This event is generated when the user begins dragging the mouse in a grid control while holding down the left button. This is generated only for header cells, immediately before the first corresponding MSG-HEADING-DRAGGED event. EVENT-DATA-1 contains the column number of the cell in which the user began dragging the mouse, and EVENT-DATA-2 contains the row number of that cell. The properties X and Y are set to match these values for the duration of this event. The properties START-X and START-Y are also set to match these values (note that unlike X and Y, these settings are retained after this event finishes).